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So im going to change the rear end gear oil.
So in the other thread regarding the draining of the rear end- the question came up- whats a suitable replacement. The manual calls for
SAE 75W-90 Synthetic Axle lube GM part # 78261 or meeting GM spec 9986115.
Well naturally, i go around looking to see who meets that spec. Seems there arent too many. Not even Mobil 1. I emailed Mobil 1 this morning. No reply yet. But I found a thread at another forum where this same issue came up.
One of the posters seems to be an engineer (im guessing GM????).
But its an interesting post- that confused them- and confuses me too as to what is the correct oil to use (aftermarket). I know that if I used something other than whats called for- id be having constant thoughts for the rest of my life- DID I USE THE WRONG FLUID?????.
At this point im very tempted to simply go to dealer and get the grape scented stuff that meets the GM spec- though Id pay quite a bit more- at least id have peace of mind regarding whether I have or do not have friction modifiers etc. etc...... I thought it would be as simple as going to auto parts and getting a bottle of gear oil.... but not that simple....
(for my 07 Impala- manual called for engine oil meeting a certain spec- no bottles listed this spec- thought website showed their spec data sheet did meet this GM spec- so i feel comy with that brand)... but I dont feel comfortable at this point with any of the gear oils- as none show them meeting that spec....
heres the very interesting thread.

“Recommended for use in differentials, manual
transmissions and other gear applications requiring any
of the following specifications: API GL-5, MT-1,
MIL-PRF-2105E, Dana SHAES 234 (Formerly Eaton
PS-037), Mack GO-J, or the differential (hypoid) gear oil
specifications from all domestic and foreign manufacturers
such as GM, Ford and Daimler Chrysler. Can also be used
in axles where an API GL-4 lubricant is recommended.”

Click on the link... https://www.amsoil.com/storefront/svg.aspx .... This is the gear oil in the TSB above. On the TSB. GM tells that they fill at the factory in G80/LS axle's 2 separate fills, a gear oil then an additive. When GM dealers do a replacement of gear oil, they use the GM part # for refill. That has the additive included for LS axles. GM owners manuals say to use GM part # OR the equivalent #9986115. The reason owners manuals, and Motor Information Systems both print the statement, “Do not add friction Modifier” is that GM specification #9986115, part #12378557 (USA) or #88901362 (Canada) have the limited slip friction characteristics already in the formula.
Now I'm not a lawyer and I don't have a Masters degree in the English language in order to play with words. But, the equivalent #9986115 for GM and the Amsoil 75W-90 (SVG), is a synthetic GL-5, MT-1 oil. Amsoil says on the web site """"Recommended for use in differentials, manual transmissions and other gear applications requiring any of the following specifications: API GL-5, MT-1, MIL-PRF-2105E, Dana SHAES 234 (Formerly Eaton PS-037), Mack GO-J, or the differential (hypoid) gear oil specifications from all domestic and foreign manufacturers such as GM, Ford and Daimler Chrysler""". IMHO and that's all it is, GM's #9986115 is plain old GL-5, MT-1 Synthetic oil that can be purchased at AUTO ZONE, NAPA or any other parts house that sells MB1, Castrol, Valvoline, RP, Redline. Look for the GL-5, MT-1 on the bottle for LS axles...................

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